220 



Prof. J. Joly. 



In my former paper I restricted my reference to colour vision to this 

 evolutionary aspect of the matter. In the present paper I have something 

 to add respecting the retinal apparatus developed to subserve the end which 

 Nature had in view. 



(1) The law defining the stimulation of excitable tissues known as the 

 " All or none " law is generally accepted by physiologists. It comes to this : 

 in order that a disturbance be set up and propagated in a nerve, the exciting 

 cause must possess a certain minimal intensity. The intensity of the trans- 

 mitted effect is a constant proper to the particular tissue. When end effects 

 — e.g., muscular contraction — exhibit different grades of intensity, this is due 

 to difference in the numbers of fibres activated by the primary stimulus. If 

 one fibre only existed in the nerve, one intensity only of end effect is 

 possible, whatever be the intensity of the exciting stimulus. There may be 

 a summation of stimuli. A stimulus too weak to activate the nerve leaves 

 behind it a local change at the point of application. A second stimulus 

 arriving before the condition induced by the first stimulus has passed away 

 may produce the effect of the minimal stimulus. 



The existence of a " refractory period " must also be noted. This appears 

 to be a period of recuperation during which the nerve fibre regains its 

 excitability. " All excitable tissues are incapable of response to a second 

 stimulus applied at a short interval of time, differing in different tissues, after 

 a previous one."* In the latter part of the refractory period stimuli stronger 

 than normal may secure excitation, but the magnitude of the disturbance 

 propagated is subnormal. The refractory state appears to act as inhibiting 

 the transmission of a too sustained and intense disturbance ; the recuperative 

 influence at the nerve terminal being, as it were, used up as fast as it is 

 generated. 



The refractory interval must vary greatly according to the nature of the 

 tissue. It may be as great as 0'002 sec. when nerves controlling muscular 

 excitation are concerned. Stimuli travelling over nerves concerned with 

 audition must succeed each other many thousands of times in a second in 

 order that note-frequency be interpreted to the brain. Thus the researches 

 of Wrightson and Keith point to four impulses per wave being necessary. 

 Middle C would, therefore, require over 1,000 impulses per second, and a note 

 of 15,000 vibrations per second (higher and audible) would require 60,000 

 impulses per secoud.-f 



(2) The bearing of these laws on the present subject is obvious. If the 



* Bayliss, ' General Physiology,' p. 433. 



t Keith in Sir Thomas Wrightsoii's ' The Analytical Mechanism of the Middle Ear,' 

 1918, p. 212. 



